Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an static electric field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets are collected in the vessel.
In the non-continuous process, or the so-called “drop-on-demand” process, a droplet is generated and expelled from the nozzle in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is ever increasing for economical reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing nanocrystalline, nanoporous inorganic compounds, preferably oxides such as aluminum oxides or silicon dioxide, or oxide/hydroxides such as aluminum oxide/hydroxides. Such recording sheets are known as “nanoporous” recording sheets.
Such recording sheets available today do not meet all of the required demands. In particular, in the case where dye-based inks are used for recording, the water fastness and the diffusion fastness of images printed on these recording sheets have to be improved. In the case where pigment-based inks are used for recording, the surface gloss of images printed on these recording sheets has to be improved, because very often the required photo quality is not obtained due to a bad compatibility between the ink-receiving layer and the ink. Particularly disturbing are gloss differences between different parts of the image. Furthermore, the manufacturing process of the known nanoporous recording sheets is not well mastered on an industrial scale.
Patent application DE 10,020,346 describes a recording sheet, which contains silicon dioxide prepared in the gas phase with a size of the primary particles of at most 20 nm, wherein the surface of the silicon dioxide has been modified by a treatment with aluminum chlorohydrate.
Patent application WO 00/20,221 describes the reaction of silicon dioxide prepared in the gas phase with aluminum chlorohydrate. The modified silicon dioxide is incorporated afterwards into an ink-receiving layer of nanoporous recording sheets for ink jet printing.
The described modification procedures show, however, the disadvantage that high quantities of aluminum chlorohydrate are needed in the modification step. Reaction speed is low and the resulting dispersions show bad storage stability due to their high salt content. The dispersions may be used only at a low value of pH, because they gel at higher values of pH. High quantities of the hardener boric acid are needed for hardening these recording sheets.
Patent application WO 02/094,573 describes the use of silicon dioxide prepared in the gas phase in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with aminoorganosilanes.
Patent application WO 01/05,599 describes the use of silicon dioxide pigments in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with cationic aminoorganosiloxanes.
Patent application EP 0,983,867 describes the use of colloidal silicon dioxide in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with silanes of general formula (R1)nSi(OR2)4-n, wherein at least one of the substituents R1 contains an amino group.
The described modification procedures show the advantage that only relatively small quantities of aminoorganosilanes are needed in the modification step. The modification reaction occurs in a relatively broad range of values of pH. Only small quantities of the hardener boric acid are needed for hardening these recording sheets due to their higher value of pH.
The described modification procedures show, however, the disadvantage that the aminoorganosilanes are expensive and that the modified dispersions fix negatively charged species only in a relatively narrow range of values of pH.